When manufacturing electronic circuits, semiconductor integrated circuits (ICs) are mounted on boards of the type commonly known as printed circuit boards. It is important in the process that the ICs be properly soldered at their terminal pins. Defects may occur whereby individual IC pins do not make the right contact.
Following assembly, the boards must be tested for proper operation. The state of the art offers two possibilities. On one hand, a full-function test may be carried out. All board inputs and outputs are connected to a test computer and all functions are thoroughly tested. With complex circuits which are typical today, such a function test has the disadvantage that it takes a very long time. Moreover, for a variety of reasons, large circuits hardly permit full-function testing any more.
Accordingly it is better to test the ICs before they are soldered to the board and merely to test for proper contacts following board assembly.
Procedures already are known by which the contacts of ICs soldered to boards are tested by checking the diode between two IC pins. The diode characteristic can be ascertained by one simple measurement and one can reliably conclude that this diode is still properly functioning and, most of all, that the two pins of the diode being measured were properly soldered.
If now these two IC pins are connected to two conducting strips of a board and these two strips are connected to a test device, an unequivocal test result must follow. But problems will arise when several diodes of one or more ICs are connected in parallel to the two conducting strips making contact with the test device. In that case, it is difficult to ascertain whether current passes through all the parallel diodes or whether, for instance, one of them remains non-conducting because of defective contact-making. This ambiguity arises because there are large manufacturing tolerances for the diode characteristics and because, by the nature of making IC diodes, they intrinsically evince characteristics with high dispersions. Where there are several parallel diodes, the measured current cannot unequivocally reveal whether all diodes were properly connected or whether one has failed on account of a contact defect, for instance. As a result, the applicability of this procedure, which is known as node-impedance measurement, is restricted to a narrow range.